The present invention is directed to a component operating with bulk acoustic waves that comprises a thin-film resonator, which may be called a bulk acoustic wave resonator (BAW resonator) or a thin-film bulk acoustic wave resonator (FBAR). The invention is also directed to a method for producing this component.
Thin-film resonators play a role as filters for terminals or end devices for mobile telecommunication.
A thin-film resonator RE is schematically shown in FIG. 1 and comprises two electrodes E1 and E2 with a piezoelectric layer PS arranged between them. The thin-film resonator RE is arranged on a carrier substrate TS. It is possible to arrange the thin-film resonator over a recess provided in the carrier substrate. The propagation in the direction of the carrier substrate of the acoustic waves generated in the resonator is thereby prevented by the recess. Another possibility, as illustrated in FIG. 1, is to arrange an acoustic mirror AS between the resonator RE and the carrier substrate TS, which prevents the escape of the acoustic waves from the resonator in the direction of the carrier substrate TS. An acoustic mirror AS comprises, for example, an alternating sequence of layers HZ, which have a high acoustic impedance and layers LZ, which have a low acoustic impedance, so that the reflection of the acoustic waves occurs at the boundary surfaces of these layers. For example, conductive layers or, respectively, metal layers can be used as mirror layers HZ with high acoustic impedance. The thickness of the mirror layers is approximately ¼ of the wavelength of the acoustic wave in the material of the layer. In order to achieve a higher possible bandwidth in the component, the number of mirror layers should be kept as low as possible. For this, the impedance jump between the layers with different impedances must be as high as possible. In particular, when a plurality of resonators are arranged on a common acoustic mirror, the structure of the conductive layers in the mirror is necessary in order to reduce parasitic electrical couplings over a contiguous conductive layer within a filter, which is comprised of a plurality of resonators. An exemplary BAW resonator is shown in FIG. 2, in which two mirror layers with high acoustic impedance are respectively fashioned as a structure mirror layer HZ1, HZ2 and the mirror layers LZ with low acoustic impedance exhibit a constant thickness throughout.
The structuring of a layer influences the subsequent processing of the layers, for example, the electrode E1, piezoelectric layer PS and the electrode E2, of a thin-film resonator lying above it, since the subsequent layers to be processed must be guided over the edges KA generated during the structuring of the first layer. The discontinuities of the structure formed by the edges KA can, as the case may be, lead to interruptions or breaks of the layers arranged above them and, thus, impair the functionality of the entire component. A sufficient edge covering is, therefore, only possible given edges fashioned flat or with a slight height differences at the edges, in comparison to the slice thickness of the covered layer. A structuring method to generate flat edges or, respectively, the use of thicker layers is not always possible, for technological reasons or reasons of design technology.
U.S. Pat. No. 6,496,085 B2, whose disclosure is incorporated herein by reference thereto and which issued from U.S. Patent Application Publication, US 2002/0084873 A1, discloses a method for generating a structured acoustic mirror in which all mirror layers are first applied on the carrier substrate and are subsequently structured together by etching. Both high-impedance layers and low-impedance layers are thereby structured. The structures fashioned are embedded in a dielectric layer that is planarized before deposition of the layers of the thin-film resonator. However, this method has the disadvantage that an elaborate dry etching or different wet etching methods for high-impedance layers or for the low-impedance layers are necessary to structure the entire stack of layers. In the application of the dielectric layer, steps are generated corresponding to the height of the structured layer stack, whose planarization is more elaborate the higher the structures of the acoustic mirror.